If you’d like to figure out how many calories you need per day, you can use this calculator to determine it. This calculation is based on the Harris-Benedict formula for findig out your basal metabolic rate (BMR), plus your active metabolic rate (AMR), which is based on your estimation of your daily physical activity.
Your Active Metabolic Rate (the answer in the calculation) is the number of calories you need to stay at your current weight. If you want to lose weight, you can either increase your level of physical activity or decrease your caloric intake by eating less. If you reduce your current caloric intake by 500 calories every day, you will lose about one pound each week. It is important not to go below 1,200 calories each day as doing so can be dangerous. (This means, avoid using crash diets)